(1) Field of the Invention
The present invention refers to a high-output, high-productivity plant for the production of plastic items, i.e. so-called xe2x80x9cpreformsxe2x80x9d, and in particular the present invention applies in a preferred manner to machines adapted for moulding at the same time a plurality of such so-called xe2x80x9cpreformsxe2x80x9d that are intended for processing by blow moulding into appropriate final containers, especially plastic bottles.
These machines for the final moulding or blow-moulding of preforms are usually defined as xe2x80x9csingle-step machinesxe2x80x9d since they are adapted to carry out, in a continuous and automatic manner, the entire process for converting the plastic material from the pelletized state thereof into the finished product, ie. the final containers.
However, it will be appreciated that the present invention shall be understood as applying also to other kinds of machines intended to only produced finished, but not blow-moulded, preforms, since the teaching of the present invention is only aimed at increasing the productivity of the plants and the related preform production processes, regardless of whether these preforms are actually produced in a single-step plant or in a plant intended exclusively for the production of preforms.
(2) State of the Art
The difference existing between single-step and two-step machines is well-known to all those skilled in the art, so that it shall not be described or explained here any further.
It is generally known that the production of preforms involves the simultaneous utilization of a plurality of injection-moulding cavities, into which a flow of resin in its molten state is injected and which remain then closed for a given period of time as required to allow the performs, so formed in the mould cavities, to appropriately cool down and, therefore, solidify. During this phase of cooling down and solidification of the preforms, the related injection moulds remain of course clamped and occupied, i.e. not available for further use, so as to allow for such a cooling down, and they cannot therefore be used for subsequent phases of injection and cooling down of further clusters of preforms. Since the various process phases that are carried out by the plant must be performed in synchrony, each one of them must be carried out within the time interval needed by the longest phase, which usually determines the so-called xe2x80x9ccycle timexe2x80x9d.
It is also a generally well-known fact that such a phase, which includes the steps of:
closing or clamping the moulds,
injecting the molten resin into the same moulds,
cooling down the injected resin and consolidating the respective preforms,
opening the moulds and
removing or knocking out the preforms from the moulds,
takes time, i.e. has a duration that is far longer than that of the other process phases carried out by the plant and, as a result, it is decisive in view of determining said cycle time and, therefore, it directly affects the actual productivity of the plant. Such a complete phase, however, can be appropriately broken down into its various, distinct sub-phases, each one of which is active, i.e. goes on for a determined respective sub-period of time.
Also generally well-known is the fact that the longest sub-period of time of the sub-phases is the one used for cooling down the preforms after the consolidation thereof. It therefore is just natural for the complete phase to be divided into an orderly sequence of several sub-phases, which the preform cooling-down sub-phase following the preform consolidation phase belongs to. This of course enables a reduction in the original cycle time to be obtained, which in this way becomes the cycle time relating to the cooling-down sub-phase. Such a solution is well-known in the art, e.g. in the disclosure of U.S. Pat. No. 4,382,905 to Valyi, and has been successively improved in a number of following patents.
However, the increasing pressure exerted in view of bringing about a further increase in the productivity of the injection moulds, along with the fact that such a cooling-down time, and therefore also the related cycle time, is anyway too long in view of the actual possibility of obtaining a satisfactory increase in productivity, has made it appropriate for the opportunity to be considered for such a preform cooling-down sub-phase following the preform consolidation to be in turn subdivided into a plurality of elementary phases. These of course need and take correspondingly reduced time lengths, with the result of a reduction in the cycle time and a corresponding increase in productivity.
However, the various attempts made in view of dividing the preform cooling-down phase by transferring the preforms from a proper mould to a succession of moulds, in which the temperature of the preforms is gradually reduced and brought down to a balanced state, have up to now failed to find any practical implementation owing to a number of reasons. These reasons are in particular the complexity of the plant that would be required for this purpose, the considerable increase in the overall size and space requirements that would ensue therefrom and, under certain circumstances, also the fact that continuously handling still relatively hot preforms from a mould to another one all along a whole sequence of moulds is quite likely to bring about a reduction in the quality of the preforms themselves and, anyway, to more or less significantly damage them.
Based on the above considerations, it is therefore desirable, and it is actually a main purpose of the present invention, to provide a plant and a respective process which are adapted to implement and carry out a preform cooling-down phase under a sensible reduction in the length of the original cycle time, are capable of doing away with the above described drawbacks, and are reliable and capable of being easily implemented using readily available and, therefore, cost-effective materials and techniques.
Such an aim of the present invention, along with further features thereof, is reached in a plant that is made and operates as recited in the appended claims.